In the conventional operation of electrolytic reduction cells which reduce alumina, Al.sub.2 O.sub.3, to aluminum, Al, the alumina is added to the cell according to a prescribed fixed time schedule.
The basic inherent disadvantage to this conventional method of controlling alumina additions to reduction cells, that is, breaking a specified area of a crusted layer of alumina into the molten cryolitic bath based on a fixed time interval cycle, is that there is no means of sensing the amount of alumina in the bath and taking corrective action.
Thus, if an excessive quantity of alumina is added over a length of time, there will be an accumulation of material, "muck", on the sides and bottom of the cathode that will eventually result in operational difficulties that decrease metal production. On the other hand, if too little alumina is consistently added to the cell, extra energy is required to operate the cell due to the increased anode overpotential, and the "anode effect" frequency which results from such underfeeding increases, lowering the metal production in all of the cells in the potline.
U.S. Pat. No. 3,583,896 to Piller, issued June 8, 1971, the disclosure of which is hereby incorporated herein by reference, describes a method for detecting "electrode upsets" in an aluminum reduction cell wherein the cell's so-called zero current interce E.sub.k is monitored. According to this method, a cell's voltage is measured at various times and current levels to determine the cell's operating characteristics. Projected zero-current intercept values, E.sub.k, are then determined for those operating conditions. That is, for each condition, an extrapolation is made to determine what the cell's theoretical voltage would be if the current were zero. From this data a determination is made of the cell's "normal" E.sub.k value. If the E.sub.k value falls below a predetermined level corresponding to that set for the particular type of cell, it is taken as an indication that the cell is entering an electrode upset whereby operating procedures may be taken to control the cell so that the electrode upset can be reduced or eliminated.
Although the technique described by Piller provides a method for determining when an electrode upset may occur, thereby permitting corrective action which may comprise feeding of the cell or causing an intentional "anode effect," the method merely provides a means for detecting the critical conditions once they occur and not for preventing them in advance.
In U.S. Pat. No. 4,425,201 to Wilson et al, issued Jan. 10, 1984, the disclosure of which is hereby incorporated herein by reference, a method for controlling alumina additions to a reduction cell, based upon a statistical analysis of the resistance values within the cell, is disclosed. This system was devised with regard to center breaker bar containing cells. Such cells receive alumina along the entire length of the breaker bar.
More recently, these breaker bars have been replaced by point feeders positioned at strategic locations above the cell. In these cells, small measured amounts of alumina are fed to the cell from any or all of the point feeders at a given time.
The nature of point feeder cells, as opposed to breaker bar cells, is such that much finer control of alumina additions is necessary. This is due to the smaller amount of alumina at each addition, as well as the multiple alumina addition points.
Thus, it would be highly desirable in the art if a system of automatic feed were available for point feeder type cells whereby through some monitoring of the cell, information regarding the need, or lack of same, for feeding the cell could be provided and the feed rate of the cell modified based upon this information.